Having a turbofan engine with reduced noise is important.
This invention relates to a turbofan engine for aircraft, the engine having reduced noise. In particular, the invention is concerned with a family of engines originally manufactured by Pratt & Whitney. In particular, these engines include the JT3D-3B, the JT3D-7 and the TF33 Engine, which includes the P-3/103, the P-5, the P-9, the P-100/100A and the P-102/102A Engines ("the JT3D family"). Such engines are commonly used on Boeing 707 aircraft, the Boeing KC135 and E3A aircraft, the Douglas DC8 aircraft and Lockheed C141 aircraft ("Aircraft").
Different techniques and systems are available to quiet an engine to reach different stages of noise reduction as defined by different governmental authorities. In particular, there are hush kits available to quiet Pratt & Whitney JT8D engines commonly used on Boeing 727 aircraft. Such an engine quieting system is the subject of U.S. Pat. No. 5,127,602 ("Batey"). The system uses a spacer and mixer to mix exhaust gas with fan air gas at the rear of the nozzle of the core engine. A cascade-type thrust reverser is used for thrust reverse conditions. Unlike the JT8D family, the JT3D family of engines are designed to exhaust fan air and turbine air separately to ambient air.
In other systems and with other engines, an annular pressure duct is used to direct fan air rearwardly to the rear zone of the exhaust nozzle. Such a system is applied on DC8 Sixty Series aircraft. In these cases, there is no internal mixing in the nozzle area of the exhaust air and duct air at the rear of the core engine. There is no common nozzle.
In some cases, such as the Douglas DC8-62/63 series aircraft and the Lockheed C141 aircraft, the fan air is directed to a plane just upstream of the turbine exhaust nozzle where it ducts to ambient air. In these cases, a common thrust reverser reverses both fan and turbine air. In other cases, such as the Boeing 707, Boeing KC135 and E3A aircraft and Douglas DC8-50/61 series aircraft, the fan air is directed only a short distance and exits to ambient air substantially forward of the turbine exhaust nozzle. In these cases where thrust reversing is desired, separate fan and turbine thrust reversers are required.
Furthermore, these engines have a bleed valve which, in certain situations of low power engine operation, particularly on approach to landing, is opened to bleed air from the core engine away from the thrust generating exit rearwardly of the core engine, perpendicular to the thrust axis of the engine. The opening of the bleed valve relieves very hot supersonic air flow and causes a substantial screeching noise and increases the noise characteristics noticeably in that operative state of the engine.
In the Applicant's experience, there is no system which exists for effectively quieting this JT3D family of engines in a manner to reduce the perceived noise level of the engines, especially low frequency jet mixing noise by at least three (3) noise decibels while maintaining the existing thrust levels.
There is a need to provide for the effective operation of such engines under appropriate thrust and operate under the appropriate noise-reduced conditions.
In particular, it is desirable to provide a turbofan engine with reduced noise and to provide a hush kit for retrofitting such engines which minimizes the disadvantages of known systems.